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Eddy Interaction Model for Turbulent Suspension in Reynolds-Averaged

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Eddy Interaction Model for Turbulent Suspension in Reynolds-Averaged Advances in Water Resources 111 (2018) 435–451 Contents lists available at ScienceDirect Advances in Water Resources journal homepage: www.elsevier.com/locate/advwatres Eddy interaction model for turbulent suspension in Reynolds-averaged T Euler–Lagrange simulations of steady sheet flow ⁎ Zhen Cheng ,1,a, Julien Chauchatb, Tian-Jian Hsua, Joseph Calantonic a Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA b Laboratory of Geophysical and Industrial Flows (LEGI), BP 53, Grenoble Cedex 9 38041, France c Marine Geosciences Division, U.S. Naval Research Laboratory, Stennis Space Center, MS 39529, USA ARTICLE INFO ABSTRACT Keywords: A Reynolds-averaged Euler–Lagrange sediment transport model (CFDEM-EIM) was developed for steady sheet Euler–Lagrange model flow, where the inter-granular interactions were resolved and the flow turbulence was modeled with a low Eddy interaction model Reynolds number corrected kω− turbulence closure modified for two-phase flows. To model the effect of Turbulent suspension turbulence on the sediment suspension, the interaction between the turbulent eddies and particles was simulated Steady sheet flow with an eddy interaction model (EIM). The EIM was first calibrated with measurements from dilute suspension Rouse profile experiments. We demonstrated that the eddy-interaction model was able to reproduce the well-known Rouse Sediment transport rate profile for suspended sediment concentration. The model results were found to be sensitive to the choice of the coefficient, C0, associated with the turbulence-sediment interaction time. A value C0 = 3 was suggested to match the measured concentration in the dilute suspension. The calibrated CFDEM-EIM was used to model a steady sheet flow experiment of lightweight coarse particles and yielded reasonable agreements with measured velo- city, concentration and turbulence kinetic energy profiles. Further numerical experiments for sheet flow sug- gested that when C0 was decreased to C0 < 3, the simulation under-predicted the amount of suspended sediment in the dilute region and the Schmidt number is over-predicted (Sc > 1.0). Additional simulations for a range of Shields parameters between 0.3 and 1.2 confirmed that CFDEM-EIM was capable of predicting sediment transport rates similar to empirical formulations. Based on the analysis of sediment transport rate and transport layer thickness, the EIM and the resulting suspended load were shown to be important when the fall parameter is less than 1.25. 1. Introduction The conventional single-phase-based sediment transport models assume the dynamics of transport can be subjectively separated into bedload Studying sediment transport in rivers and coastal regions is critical and suspended load (e.g., van Rijn, 1984a; 1984b). While the suspended to understand the fluvial geomorphology, loss of wetland, and beach load are directly resolved, the bedload are parameterized by empirical erosion. For example, significant engineering efforts were devoted to formulations. Several laboratory measurements of sheet flow with the control the river discharge and sediment budget to reduce the loss of full profile of sediment transport flux and net transport rate indicated Louisiana wetland (Allison et al., 2012; Mossa, 1996). In the Indian that the split of bedload and suspended load may be too simple because River inlet, significant erosion of the north beach is mitigated through sediment entrainment/deposition is a continuous and highly dynamic proper beach nourishment that interacts with littoral drift process near the mobile bed (e.g., O’Donoghue and Wright, 2004; Revil- (Keshtpoor et al., 2013). The characteristics of sediment transport vary Baudard et al., 2015). In sheet flow, the two prevailing mechanisms significantly with sediment properties and flow conditions, and it is driving the sediment transport are inter-granular interactions and tur- widely believed that sheet flow plays a dominant role in nearshore bulent suspension (Jenkins and Hanes, 1998; Revil-Baudard et al., beach erosion and riverine sediment delivery, especially during storm 2015). In order to model the full profile of sediment transport, both and flood conditions, respectively. mechanisms must be taken into account. In the past decade, many Sheet flow is an intense sediment transport mode, in which a thick Eulerian two-phase flow models have been developed for sheet flow layer of concentrated sediment is mobilized above the quasi-static bed. transport in steady (Jenkins and Hanes, 1998; Longo, 2005; Revil- ⁎ Corresponding author. E-mail address: [email protected] (Z. Cheng). 1 Now at Applied Ocean Physics & Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA. https://doi.org/10.1016/j.advwatres.2017.11.019 Received 15 April 2017; Received in revised form 31 October 2017; Accepted 19 November 2017 Available online 21 November 2017 0309-1708/ © 2017 Elsevier Ltd. All rights reserved. Z. Cheng et al. Advances in Water Resources 111 (2018) 435–451 Baudard and Chauchat, 2013) and oscillatory flows (Dong and Zhang, a Lagrangian approach. 2002; Hsu et al., 2004), Amoudry et al.,(Chen et al., 2011; Cheng et al., In the stochastic Lagrange model for particle dispersion, the tur- 2017a; Liu and Sato, 2006). By solving the mass and momentum bulent agitation to the sediment particles are considered either through equations of fluid phase and sediment phase with appropriate closures a random-walk model (RWM) or an eddy interaction model (EIM). In for interphase momentum transfer, turbulence, and intergranular the RWM framework, the strength of particle velocity fluctuations are stresses, these models are able to resolve the entire profiles of sediment typically assumed to be similar to the fluid turbulence, and a series of transport without the assumption of bedload and suspended load. random velocity fluctuations are directly added to the particle velo- In the continuum description of the sediment phase, the assumption cities. While the Lagrange model with RWM is successful in studying of uniform particle properties and spherical particle shapes are usually the particle dispersion in mixing layer (Coimbra et al., 1998) and dilute adopted. To better capture the polydisperse nature of sediment trans- suspension (Shi and Yu, 2015), the assumption of estimating the par- port and irregular particle shapes, the Lagrangian approach for the ticle velocity fluctuations based on the fluid turbulence is crucial, and particle phase, namely the Discrete Element Method (DEM, Cundall and many researchers found that the correlation between the particle and Strack, 1979; Maurin et al., 2015; Sun and Xiao, 2016a) is superior to fluid fluctuations are highly dependent on the particle Stokes number, the Eulerian approach because individual particle properties may be St= tpl/ t (Balachandar and Eaton, 2010), where tp is the particle re- uniquely specified (Calantoni et al., 2004; Fukuoka et al., 2014; Harada sponse time, and tl is the characteristic time scale of energetic eddies. and Gotoh, 2008). One of the main challenges in modeling sheet flow For the particles with very small inertia (St ≪ 1), they can closely follow arise from the various length scales involved in inter-granular interac- the eddy motion. However, if St ≫ 1, the particle trajectory is hardly tions and sediment-turbulence interactions. To resolve the flow turbu- affected by the fluid eddy motion. Due to the particle inertia effect, it lence and turbulence-sediment interactions in sheet flow, the compu- was found that the fluid turbulent intensity needs to be enhanced for tational domain needs to be sufficiently large to resolve the largest medium to coarse particles (Shi and Yu, 2015). This problem can be eddies, while the grid resolution should be small enough to resolve the largely remedied by the EIM (Matida et al., 2004), where the fluid energy containing turbulent eddies. This constrain becomes even more velocity fluctuations associated with the fluid turbulence are added challenging in the Euler–Lagrange modeling framework. Large domains through the particle-sediment interaction force, i.e., the drag force. This require both a large number of grid points to resolve a sufficient approach incorporates the particle inertia effect naturally and it is ap- amount of turbulence energy cascade (i.e., large-eddy simulation) and a plicable for a wide range of sediment properties. Graham (1996) found large number of particles in a given simulation (e.g., Finn et al., 2016). that the dispersion of inertial particles may be correctly represented It is well-established that in sheet flow, the transport layer thickness with a suitable choice of maximum interaction time and length scales scales with the grain size and the Shields parameter (Wilson, 1987), with the eddies. This model was later improved by using a randomly suggesting that a common sheet flow layer thickness must be about sampled eddy interaction time, in which more realistic turbulent scales several tens of grain diameters. To simulate the largest eddies and their become possible, and the enhanced dispersion of high-inertia particles subsequent cascade, the domain lengths in the two horizontal directions are captured. In the previous studies of particle dispersion (e.g., Shi and must be proportional to the boundary layer thickness, which is usually Yu, 2015), the turbulent intensity is either prescribed from the em- about several tens of centimeters. For a bed layer thickness of 50 grains pirical formula, or modeled using clear fluid turbulence closure without with
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